To meet the need for ever-increasing information capacity in wireless communication systems, research efforts have recently turned to the physical layer to increase spectral efficiency. One aspect of this research relevant to this invention is in the area of multiuser receivers. These receivers seek to minimize interference between mutual users of a spread spectrum wireless system, and generally include multiuser detectors, linear decorrelators, and linear minimum mean-square-error (MMSE) receivers. An important distinction between multiuser receivers and their matched-filter counterparts is that multiuser receivers delve into the structure of interference among disparate (simultaneous) users or the system in order to demodulate the signal of one particular user of interest.
Certain prior art receivers combat different types of interference to increase either information-carrying capacity (traditional capacity, such as defined by the Shannon limit), or user capacity, the latter being the maximum number of users from which a multiuser receiver may reliably demodulate the intended signal. These prior art receivers rely on the difference in user power levels being within a quite narrow range in order that stronger signals not produce excessive interference to weaker signals.
Regardless of the practical application of the above receivers to cellular communications where effective power control among users is a viable assumption, there exist wireless systems where that assumption does not hold, and where Doppler effects are so great that power control for every burst is not practical, or where a signal must reach a receiver more remote than the one interfered with. Such a system 20 is shown in FIG. 1.
FIG. 1 depicts a series of nodes communicating with one another over a wireless network, preferably secure. Assume a first node 22 carries a multiuser receiver, and wishes to receive communications from any of the other nodes. A second 24 and third 26 node are nearest to the first, and traditional power control is not necessarily impractical as between them and the first node 22. A fourth 28 and fifth 30 nodes are located proximal to one another as compared to the first node 22, but are moving in different directions at high speed. Doppler effects are opposed and not negligible. Direct power control to the extent achieved in cellular communications is impractical for them, because each node 28, 30 changes its distance from the first node 22 rapidly, and power control cannot keep up with the spatial changes between nodes. As the fourth node 28 closes on the first node 22, its signal carries a higher power level that will obscure the signal from the fifth node 30, whose power level as seen by the first node 22 is receding. Traditional open/closed loop power control will lag behind the power levels of the various users seen by the first node 22, essentially blinding it to at least some of the transmissions from the other nodes.
Anticipatory power control based on expected relative position might resolve the above problem were the system 20 a cellular one. Traditional power control presumes that only a single node need receive transmission from multiple users. In that model for example, a cellular base station uses a multiuser receiver and individual cellular users each use a RAKE receiver. But where the system 20 of FIG. 1 is a mesh network, any of the various nodes may include a multiuser receiver to receive transmissions from any other node. In that instance, anticipatory power control may work to constrain received signals to within a narrow power range for one of the nodes, but will expand, even beyond a range that would exist in the absence of power control, the power range for those same signals when they are received by other nodes (depending upon the location of those other nodes).
The above is generally termed in the art as a near-far power imbalance. In the system 20 of FIG. 1, the difference in power for various signals received at the first node 22 may be several orders of magnitude. At any given instant, the first node 22 may seek to receive a signal from what it sees as the strongest fourth node 28 and a weakest seventh node 34. A sixth node 32 may simultaneously seek to receive a signal from what is sees as an intermediate strength fifth node 30 and a weakest second node 24. What is needed in the art is a multiuser receiver that can operate reliably in such an environment. Specifically, what is needed is a multiuser receiver and method that can resolve any of several received user signals whose power levels span at least an order of magnitude range, and preferably greater.